[flutter_tools] resolve requests to web directory in debug mode (#51995)

dev/bots/test.dart

@@ -588,6 +588,7 @@ Future<void> _runWebIntegrationTests() async {
   await _runWebStackTraceTest('profile');
   await _runWebStackTraceTest('release');
   await _runWebDebugTest('lib/stack_trace.dart');
+  await _runWebDebugTest('lib/web_directory_loading.dart');
   await _runWebDebugTest('test/test.dart');
 }
 

dev/integration_tests/web/lib/web_directory_loading.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:html' as html;
+
+// Attempt to load a file that is hosted in the applications's `web/` directory.
+Future<void> main() async {
+  try {
+    final html.HttpRequest request = await html.HttpRequest.request(
+      '/example',
+      method: 'GET',
+    );
+    final String body = request.responseText;
+    if (body == 'This is an Example') {
+      print('--- TEST SUCCEEDED ---');
+    } else {
+      print('--- TEST FAILED ---');
+    }
+  } catch (err) {
+    print(err);
+    print('--- TEST FAILED ---');
+  }
+}

dev/integration_tests/web/web/example

+This is an Example
\ No newline at end of file

packages/flutter_tools/lib/src/build_runner/devfs_web.dart

@@ -190,10 +190,16 @@ class WebAssetServer implements AssetReader {
     // Try and resolve the path relative to the built asset directory.
     if (!file.existsSync()) {
       final Uri potential = globals.fs.directory(getAssetBuildDirectory())
-        .uri.resolve( requestPath.replaceFirst('/assets/', ''));
+        .uri.resolve(requestPath.replaceFirst('/assets/', ''));
       file = globals.fs.file(potential);
     }
 
+    if (!file.existsSync()) {
+      final String webPath = globals.fs.path.join(
+        globals.fs.currentDirectory.childDirectory('web').path, requestPath.substring(1));
+      file = globals.fs.file(webPath);
+    }
+
     if (!file.existsSync()) {
       return shelf.Response.notFound('');
     }

packages/flutter_tools/test/general.shard/web/devfs_web_test.dart

@@ -191,6 +191,21 @@ void main() {
     ]));
     expect((await response.read().toList()).first, source.readAsBytesSync());
   }));
+  test('serves files from web directory', () => testbed.run(() async {
+    final File source = globals.fs.file(globals.fs.path.join('web', 'foo.png'))
+      ..createSync(recursive: true)
+      ..writeAsBytesSync(kTransparentImage);
+    final Response response = await webAssetServer
+      .handleRequest(Request('GET', Uri.parse('http://foobar/foo.png')));
+
+    expect(response.headers, allOf(<Matcher>[
+      containsPair(HttpHeaders.contentLengthHeader, source.lengthSync().toString()),
+      containsPair(HttpHeaders.contentTypeHeader, 'image/png'),
+      containsPair(HttpHeaders.etagHeader, isNotNull),
+      containsPair(HttpHeaders.cacheControlHeader, 'max-age=0, must-revalidate')
+    ]));
+    expect((await response.read().toList()).first, source.readAsBytesSync());
+  }));
 
    test('serves asset files from in filesystem with known mime type on Windows', () => testbed.run(() async {
     final File source = globals.fs.file(globals.fs.path.join('build', 'flutter_assets', 'foo.png'))